1. Field of Invention
The present invention relates to power-line-operated inverter-type fluorescent lamp ballasts, particularly of the kind using a current-excited parallel-loaded resonant L-C circuit for matching the inverter output to the lamp load.
2. Prior Art
Power-line-operated inverter-type fluorescent lamp ballasts using a current-excited parallel-loaded resonant (or near resonant) L-C circuit for matching the inverter output to the lamp load are well known and widely used. An example of such a ballast is described in U.S. Pat. No. 4,277,726 to Burke.
One significant problem generally associated with these ballasts relates to their relatively poor ability to regulate lamp light output in response to variations in the magnitude of the power line voltage.
An object of the present invention is that of providing cost-effective means for controlling and/or regulating the light output associated with inverter-type fluorescent lamp ballasts.
This as well as other important objects and advantages of the present invention will become apparent from the following description.
A push-pull inverter is supplied from an inductively current-limited DC voltage source by way of a center-tap on a transformer having significant inductance. This transformer inductance is parallel-coupled to a capacitance means. A fluorescent lamp is series-connected with a current-limiting inductor, and this lamp-inductor series-combination is connected in parallel with the capacitance means.
The inverter is made to self-oscillate through positive feedback provided by way of a saturable current transformer. The inverter frequency is determined by the saturation time of this current transformer, which saturation time is designed to be somewhat longer than the half-cycle period of the natural resonance frequency of the transformer inductance means combined with the parallel-coupled capacitance means as well as the parallel-coupled lamp-inductor series-combination.
The saturable current transformer comprises a ferrite magnetic core, and the length of the saturation time of this transformer is determined by the magnitude of the magnetic saturation flux of this ferrite core.
The magnitude of the magnetic saturation flux is determined by the temperature of the ferrite magnetic core: the higher the temperature, the lower the magnitude of the magnetic saturation flux. By heating the ferrite core as a function of the magnitude of the DC voltage supplying power to the inverter, the inverter frequency is made to vary in inverse relationship with the magnitude of this DC voltage. As an overall result, lamp light output is kept substantially constant in spite of substantial variations in the magnitude of the DC supply voltage.